In the manufacture of CRTs, it is often desirable to have the effective faceplate transmission at about 40% to enhance the contrast of the displayed images. Essentially transmissions around 40% effectively reduce the light noise from ambient sources in the vicinity of the tube to help prevent the ambient sources from interfering with the quality of the displayed images. One means of making CRTs with such low transmission is to use dark glass; however, the manufacture of dark glass is more expensive than the light, high transmission glass. As such, the CRT industry has been utilizing neutral density faceplate coatings on faceplates to effectively decrease the transmission. This effort has proven to be more cost effective.
The recent trend is that CRT designers and manufacturers prefer, for a given size tube, that all faceplates start out as having a specific, high transmission. The reasoning is two fold. First, as alluded to above, such glass costs less because there is less tinting materials in the glass. The second reason relates to the fact that manufacturers often need to manufacture CRTs having several different transmissions within a given size. Thus, a manufacturer can simply tailor the transmission of the faceplate with appropriate contrast enhancing faceplate coatings to meet the varying faceplate transmission demands.
In U.S. Pat. No. 5,750,187, the key components of a contrast enhancing faceplate coating were lithium polysilicate and carbon particles. In specific examples in that patent, the faceplate transmission was decreased in the range of 19–37% with respect the uncoated faceplates, while the gloss of the coated faceplates was in the range of 56–70 as measured by a 60° gloss measuring technique. Although the capability of reducing the faceplate transmission with faceplate coatings is deemed important, it is likewise important in certain markets to have the capability to simultaneously retain high gloss values in a cost effective manner. Therefore, the CRT industry is challenged to produce low cost CRTs, wherein faceplates have a coating with high gloss values and significant transmission reducing properties. Specifically, the coating should be able to reduce the transmission of the faceplates by about 50% and simultaneously allow the faceplates to have gloss values around 70–90 as measured by a 60° gloss measuring technique.